The present invention relates to a wind tunnel balance that permits high-precision measurements; in particular, the wind tunnel balance according to the invention can be used to separately detect parasitic forces in the z direction that are produced, for example, due to the vehicle being incorrectly affixed to a conveyor belt. In addition, the weight forces acting on a platform are minimized, thus increasing the measurement precision. This can be achieved by supporting the platform of the wind tunnel balance so that it is able to move relative to a frame; the weight force of a belt unit and a (motor) vehicle resting on it are supported relative to the frame.
In order to measure forces that act on a motor vehicle during aerodynamic testing in a wind tunnel, various test bed or test stand setups are known, e.g. one-belt, three-belt, or five-belt systems. The coordinate system is selected so that the x axis is oriented in the travel direction of a conveyor belt of the test stand. The y axis is perpendicular to the conveyor belt. The z axis, starting from the surface of an upper run of the conveyor belt, points away from the conveyor belt (see the coordinate system in the drawings).
One-belt systems have a belt unit with a wide, continuous conveyor belt that travels around two rollers or drums. For aerodynamic tests, the motor vehicle is positioned with all four wheels on the one conveyor belt and is affixed relative to it. This is accomplished, for example, with fixing devices that are situated to the side of the conveyor belt. The wheels are supported on mounts (rocker pads), which are positioned underneath the upper run of the conveyor belt, between the rollers. As a rule, the rocker pads are connected to measuring sensors so that it is possible, for example, to detect forces in the z direction, e.g. the weight force of the motor vehicle, lift forces, and negative lift forces. The forces in the z direction are therefore measured through the conveyor belt. In this context, this is referred to as a through-the-belt measurement.
Through-the-belt measurement is also used in three-belt, five-belt, and seven-belt systems. In three-belt test stands, the tires of the motor vehicle rest on narrower side belt units. In order to simulate the road surface, a so-called middle belt travels between the conveyor belts of the side belt units. The through-the-belt measurement is carried out on the two side belt units. In a five-belt test stand setup, a middle belt unit and four smaller belt units to the side of it are provided. Usually, a motor vehicle is placed onto the four side belt units and the belt units themselves are weighed. In this case, the through-the-belt measurement is frequently not used, but is, however, possible. In five-belt systems, the through-the-belt measurement is particularly used when the track width of a motor vehicle is less than or equal to the width of the middle belt and the motor vehicle is resting on only the middle belt.
In known wind tunnel balances, the measurement of aerodynamic forces that act on a (motor) vehicle when it is being tested on the wind tunnel balance can be distorted by parasitic forces. These parasitic forces are caused among other things by slippage of the wheels of the vehicle, an imprecise fixing of the vehicle to the conveyor belt, or due to so-called walk and/or roll forces. Parasitic forces in the z direction are, for example, produced due to the fact that securing cables of a restraint system (fastening device) that holds the vehicle in position on the conveyor belt are not oriented parallel to the horizontal and therefore generate a parasitic (fictitious) force that is detected by the rocker pads. Conventional wind tunnel balances cannot separately detect these parasitic forces or can only do so with a considerable degree of additional complexity, which can result in further measurement inaccuracies.
With regard to the topic of parasitic forces, EP 1 656 541 proposes placing an entire wind tunnel test bed with a single conveyor belt on a single weighing plate that is supported so that it can be moved relative to a stationary reference system. Aerodynamic forces are detected by means of movements of the weighing plate relative to the stationary reference system. In this apparatus, the parasitic forces in the x, y, and z directions occur as internal forces so that it is not possible to display or separately detect them. Consequently, one disadvantage of the test bed according to EP 1 656 541 is that parasitic (fictitious) forces that are produced, for example, by the fixing of the vehicle cannot be easily displayed or detected separately from the aerodynamic forces in the z direction (which are produced by aerodynamic lift and negative lift as well as the vehicle weight). This can lead to distortions in the measuring results. The device according to EP 1 656 541 also does not permit ventilation losses of the vehicle wheels to be detected separately (in isolation). It is also disadvantageous that the wind tunnel balance according to EP 1 656 541 rests with its whole weight on the single weighing plate that is simultaneously also used to detect the forces. The high weight that is consequently also resting on the weighing plate can reduce the measurement precision.